Tensioning devices are used as a control device for a power transmission chain, or any similar power transmission devices, as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises, slippage, or the unmeshing of teeth (in cases of a toothed belt). Prevention of such slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because slippage will throw off the camshaft timing by several degrees, possibly rendering the engine inoperative or causing damage. However, in the harsh environment in which an internal combustion engine operates, belt tension can vary between excessively high or low levels as a result of the wide variations in temperature and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. Moreover, wear to the chain components, during prolonged use, can result in a decrease in the tension of the chain. In addition, cam shaft and crank shaft induced torsional vibrations cause belt tension to vary considerably. This tension variation results in chain elongation, determined by belt stiffness.
Generally, blade-type chain tensioners utilize a blade spring interlocked under tension with a shoe to provide tension to a chain. Specifically the blade spring is arcuate in shape and the shoe is relatively flat. The shoe further is constructed from a material which will deform or "creep" upon experiencing a load at a high temperature. The blade spring is bent to correspond to the shape of the shoe and interlocked therewith. Through the bias of the blade spring, which wishes to return to its original, more arcuate shape, a load is applied to the shoe. Thus as the operation of the engine causes the temperature of the shoe to increase, the load from the blade spring causes the shoe to deform to a more arcuate shape. Through such deformation tension is provided to a chain. Specifically, the blade assembly is positioned relative to the chain so that as the shoe becomes more arcuate it bears upon a span of chain thereby increasing chain tension. The present invention concerns a blade-type chain tensioner having an improved mounting for the blade spring to the shoe.
Past tensioners have mounted the blade spring to the shoe through a pin and slot structure. Turner et al. U.S. Pat. No. 3,490,302 entitled "Chain Tensioner" discloses a blade assembly having a shoe made of a material that will "creep" under a load at an elevated temperature. The load to the shoe is provided by a blade spring continuously bearing against the shoe. The blade spring is mounted to mechanically interlock with, and thereby provide a load to, the shoe through a hole and pin combination. Specifically, the blade spring has a triangular aperture at one end that must be aligned with a triangular projection on the shoe. When the triangular projection of the shoe mates with the triangular aperture of the blade spring, a pin is inserted into a bore in the projection to mechanically interlock the blade spring and the shoe.
Another construction for mounting a blade spring to a shoe is shown in Young et al. U.S. Pat. No. 4,921,472 entitled "Chain Tensioner," incorporated herein by reference. This reference discloses a blade-type tensioner having a blade spring mechanically interlocked with a shoe through a passageway in the end of the shoe. Specifically, the shoe has a passageway having a shoulder and a chamber into which the blade spring fits. This configuration allows a first end of the blade spring to be slid through the passageway, at one end of the shoe, until it engages a chamber at the other end of the shoe. The second end of the blade spring is secured in tension to the shoe by a shoulder in the passageway. Slots cut into each end of the blade spring may further be used to secure the blade spring in the shoe. Through such a configuration, the blade spring is mechanically interlocked with the shoe by only the geometrical configuration of the parts.
A further construction for mounting a blade spring to a shoe is disclosed in Cradduck et al. U.S. Pat No. 5,055,088 entitled "Spring Blade Chain Tensioner," incorporated herein by reference. This reference discloses a blade-type tensioner which utilizes a plurality of blade springs interlocked with a plastic shoe through a passageway in the shoe and fastened using a pin.
These prior art blade-type chain tensioners, however, have used blade springs having complicated shapes or required the use of separate fasteners, such as pins, or both to interlock the blade spring and the shoe. These features add to the cost of manufacturing and installing the blade spring to the shoe. Moreover, any cutouts in the blade spring may create a stress riser i.e., an area of stress concentration, which could adversely impact the reliability of the chain tensioner. In addition, such cut-outs require additional steps in the manufacturing process and thus increase manufacturing expenses.
Accordingly, it is an object of the present invention to provide a blade-type chain tensioner in which a blade spring is mounted to a shoe and provides a load thereto without the use of any cut-outs in the blade spring or any separate fasteners.